The invention relates to scintillation cameras and positron emission tomography (PET) scanners such as are used in nuclear medicine to produce images of a portion of a patient's body. The invention more particularly relates to the detector structure of such cameras and scanners. In its most immediate sense, the invention relates to the manner in which the photomultiplier tubes are mechanically and optically secured in the detector.
In the past, photomultiplier tubes ("PMTs") have been coupled to the light pipe or to the scintillator using grease. While this produces a coupling which is satisfactory from the standpoint of optical coupling characteristics, it is mechanically unsatisfactory and mechanical means such as spring loading were required to make sure that the PMTs were properly maintained in proper position with the light pipe.
To improve on this coupling technique, Siemens Gammasonics, Inc. has used room temperature vulcanizing ("RTV") adhesive between the primed light pipe and the PMTs. While this has been an improvement over the use of grease, it has caused another problem to come about.
This problem is that it is at least difficult, and sometimes impossible, to remove a single PMT from the light pipe. Where this can be done in the field, special tools are necessary; where this is impossible in the field, this operation must be carried out in the factory. The task is difficult because the PMTs are packed densely and the RTV bonds quite strongly (more than 100 psi in shear) to the PMT and lightpipe.
PMTs fail and must be replaced. It would therefore be advantageous to make it easier to remove a PMT without adversely affecting the optical and mechanical connection which is achieved using RTV adhesives.
One object of the invention is to provide a technique whereby PMTs may, in scintillation cameras and PET scanners, be bonded with adequate optical and mechanical characteristics while still being removable with reasonable effort and without damaging the PMT or the component to which it is attached.
Another object is, in general, to improve generally on existing PMT bonding techniques.
In accordance with the invention, the bonding adhesive used cures to a soft state. Furthermore, at least one stress-increasing body is introduced into the bonding adhesive before the PMT is glued down.
The adhesive is sufficiently strong and sufficiently transparent to ultraviolet radiation so that the mechanical and optical characteristics of the bond are entirely adequate. However, the stress-increasing bodies weaken the bond so that it is quite weak (decoupling only requires a torque of, e.g. 25 inch-pounds) in torsional load. It is therefore easy to decouple the PMT. Where the PMT is round, this may be done by grasping it and twisting it; after a minute or so of twisting, the PMT comes off without breaking. Where the PMT is square or hexagonal, the PMT may be tipped a few degrees and held in the tipped position for a few minutes. In both instances, the PMTs begin to decouple at a stress-increasing body and the decoupled interface spreads until the tube may easily be lifted off. A new PMT may then be installed in the space with a small quantity of fresh adhesive.
Advantageously, the adhesive is a silicone dielectric gel and the bodies are glass or plastic spheres which may be as small as 0.25 mm or as large as 3 mm in diameter, but which are advantageously 1 mm or 0.5 mm in diameter. Such beads are inexpensive, do not interfere with the curing of the gel, and have a highly constant diameter. Alternatively, the adhesive may be an RTV adhesive weakened with a diluent such as dimethylsiloxane, or may be a single-component adhesive. Where the PMT is bonded to an acrylic plastic light pipe, the light pipe is advantageously primed before the adhesive is applied.